I've experimented with this by refreshing the page on various subreddits and downvoting an ad. I continue to be shown an ad after I downvote it. This means those impressions are being wasted on me, as someone who has explicitly communicated that I'm uninterested in that ad.

If this is correct then I guess people like you and I are paying for (what is basically) pointless advertising.. However anywhere else (google adwords) it will show you adverts 1,000's of times even if you never click.. Simply because they still assume you are interested from your searches. Subreddits working the same way.